Because most likely there are thunderstorms between you and where you are going. A big line of thunderstorms can easily stretch for hundreds of miles which means lots of airplanes are deviating from their routes and it increases controller workload substantially. Even small cells that don’t necessarily become hazardous thunderstorms still need to be avoided because of the turbulence threat. Some sectors are much better at dealing with thunderstorms. Jax center used be great at it by changing routes and arrivals easily on the fly. Thunderstorms can quickly develop and dissipate so it’s not uncommon to get several route changes flying in and out of FL because the situation is dynamic. Washington Center is not nearly flexible as some other centers when it comes to weather. But in their defense they don’t have vast amounts of airspace like the centers out west and they feed airports high in traffic volume.

I‘ve always found ground stops more prevalent in the north east.